1. Field of the Invention
The present invention relates generally to an apparatus and method for efficiently transmitting/receiving scheduling information in a mobile communication system supporting an uplink packet data service. In particular, the present invention relates to an apparatus and method for performing soft combining on scheduling signals or ACK/NACK signals received from a plurality of cells in the case where a user equipment (UE) is located in a handover region.
2. Description of the Related Art
A Wideband Code Division Multiple Access (WCDMA) system uses an Enhanced Uplink Dedicated transport Channel (EUDCH). A EUDCH service has been proposed to improve transmission performance of uplink packet data in the WCDMA system. In the EUDCH service, transmission approval/disapproval of uplink packet data and a possible upper limit of a data rate for each UE are determined by a Node B. The information determined by the Node B is transmitted to the UE as a scheduling command. In response, the UE determines a rate of EUDCH data according to the scheduling command, and transmits data to the Node B through an uplink at the determined data rate. Further, in the EUDCH service, a Hybrid Automatic Retransmission Request (HARQ) technique is used. Therefore, the Node B determines whether it has succeeded or failed in decoding data received from the UE, and transmits an acknowledge (ACK) signal or a negative acknowledge (NACK) signal to the UE depending on the determination result.
FIG. 1 is a concept diagram for a description of a scheduling operation performed by a Node B in a mobile communication system supporting a EUDCH service. In FIG. 1, reference numeral 100 represents a Node B supporting the EUDCH service, and reference numerals 110 to 116 represent UEs transmitting EUDCH data.
If a data rate of a UE increases, reception power at which the Node B receives data from the UE increases. Therefore, a Rise over Thermal (RoT) of the UE occupies a larger portion of the total RoT. However, a decrease in data rate of a UE reduces reception power at which the Node B receives data from the UE. In this case, a RoT of the UE occupies a lesser portion of the total RoT. Therefore, the Node B performs scheduling on EUDCH packet data by taking into consideration a relation between the data rate and radio resources, and a data rate requested by the UE. The Node B limits the amount of uplink signal that it can receive while guaranteeing its reception performance. Equation (1) below illustrates the amount of uplink signal that the Node B can receive while guaranteeing its reception performance.RoT=Io/No  (1)
In Equation (1), Io denotes the full reception band's power spectral density of the Node B, and No denotes a thermal noise power spectral density of the Node B. Therefore, the RoT indicates available radio resources that the Node B can allocate for the EUDCH packet data service in an uplink.
As described above, the Node B allocates data rates to UEs such that a measured RoT of the Node B should not exceed a target RoT. That is, the Node B can allocate a lower data rate to a UE located at a long distance and a higher data rate to a UE located at a short distance.
The UEs transmit packet data with different uplink channel transmission power according to their distances from the Node B. That is, the UE 110 that is located the farthest from the Node B 100 transmits packet data with the highest transmission power 120 for uplink channels, and the UE 114 that is located the nearest to the Node B 100 transmits packet data with the lowest transmission power 124 for uplink channels. Transmission power and packet data rates applied in the UEs can be changed according to a scheduling algorithm applied in the Node B.
FIG. 2 is a diagram illustrating a basic procedure needed between a Node B 200 and a UE 202 for packet data transmission through a EUDCH. Referring to FIG. 2, in step 204, a EUDCH is set up between the Node B 200 and the UE 202. The step 204 includes a process of transmitting/receiving messages over a dedicated transport channel. Thereafter, in step 206, the UE 202 transmits information on a needed data rate and other information, based on which an uplink channel condition can be determined, to the Node B 200. The information, based on which an uplink channel condition can be determined, includes transmission power of an uplink channel that the UE 202 transmits, and a transmission power margin of the UE 202.
Upon receiving information on the uplink channel transmission power, the Node B 200 can estimate an uplink channel condition by comparing the transmission power with the reception power of the uplink channel. If a difference between the uplink channel transmission power and the uplink channel reception power is small, the Node B 200 determines that the uplink channel condition is good. However, if the difference between the transmission power and the reception power is large, the Node B 200 determines that the uplink channel condition is bad.
In the case where the UE 202 transmits its transmission power margin, the Node B 200 can estimate an uplink transmission power by subtracting the transmission power margin from the possible maximum transmission power of the UE 202 that is already known to the Node B 200. The Node B 200 then determines a possible maximum data rate for an uplink packet channel of the UE 202 or determines whether to increase or decrease a next data rate of the UE 202, using the estimated uplink channel condition of the UE 202, information on a data rate needed by the UE 202, and information on the total power of an uplink interference signal received from the UE 202. The information on the determined possible maximum data rate and increase/decrease in the next data rate is provided to the UE 202 as a downlink scheduling signal in step 208.
The UE 202 then determines a data rate for its transmission packet data according to the notified possible maximum data rate or rate increase/decrease command, and transmits the packet data to the Node B 200 at the determined data rate in step 210. The Node B 200 then receives the packet data and decodes the received packet data. If the Node B 200 succeeds in decoding the packet data, it transmits an ACK signal to the UE 202 in step 212, and the Node B 202 then transmits new packet data to the Node B 200 in the manner described in the step 210. However, if the Node B 200 fails in decoding the packet data, it transmits a NACK signal to the UE 202 in step 212. In this case, the UE 202 retransmits the data transmitted in the step 210.
As described above, when a UE is located in a soft or softer handover region, power control for the UE is achieved through pilot signals on dedicated physical control channels (DPCCHs) received from cells in an active set. That is, the power control is performed based on a signal-to-interference-plus-noise ratio (SINR) determined after soft-combining the pilot signals.
As a result, a scheduling signal and an ACK/NACK signal transmitted by each cell for a EUDCH packet are transmitted at a lower power as compared with the case where the UE is not located in a handover region. Therefore, the UE cannot guarantee reliability for a scheduling signal and an ACK/NACK signal from each cell.
Accordingly, a need exists for a system and method for receiving cell group information, and combining scheduling signals or acknowledge/negative-acknowledge (ACK/NACK) signals to improve performance.